1. Anantiserum which specifically protects mice against a virulent culture (M variant) of the hemolytic streptococcus contains specific opsonin. Phagocytosis of the organisms can be observed in the peritoneum of the protected mouse.
2. An antiserum prepared by injecting an animal with the living M variant specifically opsonizes both the F and the M variant of the strain.
3. Evidence is presented which indicates the probable identity of the specific opsonin and the anti-M precipitin of Lancefield (7). Agglutination appears to be dependent upon a different antibody.
4. It is possible to type the hemolytic streptococci by means of specific opsonins, and the opsonic method has certain advantages over agglutination, precipitation, and mouse protection tests. It is evident from what little has been done that there are many types.
5. The serum of infants contains no opsonin for the virulent hemolytic streptococci, but the serum of adults may contain specific opsonins for certain strains. Inasmuch as no opsonins were demonstrable in two polyvalent antibacterial sera examined, the possibilities of therapeutic transfusion are discussed.
